The present invention relates to a pressurized water nuclear reactor power plant and specifically to an improved sparger system for use in the pressurizer relief tank for use in such plants which protects the plant from overpressure and thus, provides safe plant operation.
In a pressurized water nuclear reactor plant, a primary coolant loop and secondary coolant loop are used to produce steam for the production of electricity. In the primary coolant loop, a pressurized fluid is passed through a nuclear reactor and, after being heated, through a line which contains a pressurizer, to a steam generator. The heated fluid enters the primary side of the steam generator which is divided into an inlet section and an outlet section by a divider plate. A tube sheet divides the steam generator into the primary side and a second side, which tube sheet has an array of holes having U-shaped heat transfer tubes inserted therein, which communicate between the inlet section and outlet section of the primary side of the steam generator. In operation, the heat pressurized fluid passes through the U-shaped heat transfer tubes and is discharged from the outlet section of the primary side of the steam generator to a line, containing a primary coolant pump, back to the reactor in a continuous closed loop. Secondary coolant is passed through the secondary side of the steam generator where it is converted into steam by heat released by the primary coolant passing through the U-shaped heat transfer tubes, which steam is used to drive a turbine to produce electricity.
The reactor coolant pressure is controlled by a pressure control system containing a pressurizer, which is a vertical, cylindrical vessel with hemispherical top and bottom heads, wherein water and steam are maintained in equilibrium by electrical heaters and water sprays. Steam can be formed by activating the heaters to increase the pressure in the primary coolant loop, or condensed by the water sprays to reduce the pressure. Power operated relieve valves and spring loaded safety valves are connected to the pressurizer and discharge to a pressurizer relief tank, where steam from the pressurizer is condensed and cooled by mixing with water.
The pressure control system for the primary loop thus includes the pressurizer and the associated sprays, heaters, power operated relief valves, safety valves, relief tank, and surge lines. This equipment is designed to accommodate changes in system volume and to limit changes in system pressure due to reactor coolant loop temperature variations during all modes of plant operation.
To reduce the problem of leakage through the valve seats, a water seal is maintained upstream of each valve seat of the pressurizer valves. The pipes connecting the pressurizer nozzles to their respective valves are shaped in the form of a loop seal. If the pressurizer pressure exceeds the set pressure of the valves, they will open, and the water from the loop seal will discharge during the accumulation period.
In order to avoid steam release to the containment from the nuclear reactor during normal and upset condition operation, the power operated relief valves (PORV) and the safety valves (SV) are thus mounted on the pressurizer and are routed to a pressurizer relief tank. The pressurizer relief tank is a partially water filled tank having a nitrogen cover gas. Hot fluids, water and steam, discharged from the power operated relief valves and safety valves are distributed into the water in the pressurizer relief tank by means of a sparger therein which conventionally is comprised of a straight conduit having a plurality of orifices therein. Typically, the sparger comprises a 12" pipe with a plurality of small orifices (about 1/2 inch diameter) in long rows no more than about 30.degree. above and below the horizontal axis of the pipe.
The number of orifices in the sparger is based on the efficient condensation of the steam flow through the sparger associated with the peak volumetric surge rate into the pressurizer during an assumed loss of load (100%) without immediate reactor trip or shutdown. Long, approximately 1" in diameter, steam plumes will be produced through each orifice in the sparger for this basic design event. For lower steam flowrates, steam could bubble out of each orifice, while for higher steam flowrates, the steam plumes, almost jets, could penetrate the water surface. For off-design flowrates, condensation efficiency of the steam in the water is reduced.
The pressure drop across the sparger is part of the total safety valve discharge system drop which is conventionally limited to a 500 pounds per square inch differential due to the limitations on the valve backpressure compensating bellows. The 500 pounds per square inch gauge bellows limit, conventionally used, is more conservative than the maximum backpressure of 750 pounds per square inch gauge normally permitted.
The design of a power operated relief valve-safety valve system that accommodates for an anticipated transient without trip (ATWT) is desired. An anticipated transient without trip is an event in which the diverse and independent reactor trip, or shutdown, safety systems fail to immediately shutdown the reactor for an extended time period. Such an event would result in a delay of the reactor trip until after the pressurizer had become completely filled with coolant water, or water solid. Following such an event, the reactor will eventually be shutdown either automatically or manually, but a delay is present. In a typical ATWT event, the insurge of reactor coolant into the pressurizer continues until and after the pressurizer is filled with water. The power operated relief valves open and relieve steam and safety valves open with the pressurizer filled with water. The safety valves discharge water for about 50 seconds while the power operated relief valves discharge water for about 120 seconds. As a result, the pressure drop across the discharge piping and sparger increases dramatically. Valve backpressure increases to approximately 1000 pounds per square inch gauge, well above both the bellows and standard test limits.
In orde to accommodate an anticipated transient without trip event, modification of existing equipment was required. According to the present invention, the discharge piping was increased to 16 inch diameter piping from standard 12" piping, and sparger modifications are provided. A simple increase in the size of the orifices in the sparger was not acceptable since steam condensation efficiency for the nominal (loss of load) design basis was insufficient. Inefficient condensation causes premature rupture of the overpressure rupture disks on the pressurizer relief tank and unnecessary discharge of such steam to the containment.
It is an object of the present invention to provide a pressure control system for a pressurized water nuclear reactor plant that will accommodate normal and anticipated transient without trip events without unnecessary discharge of steam to the containment.
It is another object of the present invention to provide an improved sparger for use in a pressurizer relief tank that provides efficient steam distribution and condensation in the pressurizer relief tank while limiting the pressure drop for normal and anticipated transient without trip events.